(1) Field of the Invention
This invention relates to a process for the preparation of a mordenite type zeolite having 5-membered rings of oxygen atoms. More particularly, it relates to a process in which a mordenite type zeolite having a high purity and a high SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio can easily be prepared with a good reproducibility without using an organic or inorganic mineralizer.
(2) Description of the Prior Art
As is seen from the fact that the term "zeolite" is derived from the Greek word "zein" (boiling stone), the zeolite is a crystalline aluminosilicate containing zeolitic water, which has an oxide molar composition represented by the following general formula: EQU M.sub.2/n.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
wherein M stands for a metal cation, n stands for the valency of the metal cation M, x is a number of at least 2, and y is a number larger than 0.
The basic structure of the zeolite comprises SiO.sub.4 tetrahedrons having four oxygen atoms at apexes with the silicon atom being as the center and AlO.sub.4 tetrahedrons having four oxygen atoms at apexes with the aluminum atom being as the center, where these SiO.sub.4 tetrahedrons and AlO.sub.4 tetrahedrons are regularly and three-dimensionally connected to one another while owning oxygen atoms jointly. A three-dimensional network structure having pores differing in the size and shape according to the manner of connection of the tetrahedrons can be provided. Negative charges of the AlO.sub.4 tetrahedrons are electrically neutralized by coupling with cations of an alkali metal or alkaline earth metal. The thus-formed pores have a size of 2 to 10 angstroms or more, and the pore size can be changed by exchanging the metal cations connected to the AlO.sub.4 tetrahedrons with other metal cations having a different size.
The zeolite is widely used as a dehydrating agent for a gas or liquid or as a molecular sieve for adsorbing and separating specific molecules by utilizing the above-mentioned pores. Furthermore, the zeolite is used on an industrial scale as a catalyst by utilizing properties as the solid acid.
The mordenite intended in the present invention is distinguishable from a natural mordenite and is ordinarily called "synthetic mordenite" or "large port mordenite". The crystal of the mordenite is of the rhombic system and is characterized by the lattice constants a, b and c of 18.1 angstroms, 20.4 angstroms and 7.5 angstroms, respectively. The pores have a channel of the 12-membered rings of oxygen atoms having a pore diameter of 6.7.times.7.0 angstroms and a channel of the 8-membered rings of oxygen atoms having a pore diameter of 2.9.times.5.7 angstroms.
The chemical composition of the mordenite has heretofore been represented by the following formula: EQU Na.sub.2 O.Al.sub.2 O.sub.3.10SiO.sub.2.6H.sub.2 O
and the mordenite is characteristic over other zeolites in the point where the silica ratio is 10 and higher than the SiO.sub.2 /Al.sub.2 O.sub.3 ratios in other zeolites. The mordenite is excellent in the heat resistance and acid resistance, and is widely used as an adsorbent or catalyst on an industrial scale.
It was found that as the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio in the zeolite is increased, the heat resistance and acid resistance are improved and the catalytic properties as the solid acid are enhanced, and therefore, high-silica zeolites have recently attracted attention in the art. Accordingly, also in case of mordenites, development of a process capable of producing a product having a higher SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio on an industrial scale is eagerly desired.
Researches have recently been made on the synthesis of a mordenite type zeolite having an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio higher than 10 (hereinafter referred to as "high-silica mordenite"), and several processes have been proposed. For example, Japanese Unexamined Patent Publication No. 55-95612 proposes a process in which neopentylamine is used as an organic mineralizer and a high-silica mordenite having an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 17 is prepared. Furthermore, the Report of Ueda, Fukushima and Koizumi (Science of Clay, Volume 22, No. 1, 1982) discloses a process in which a high-silica mordenite having an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 25.6 is prepared by using a quaternary ammonium salt as an organic mineralizer. These processes, however, are not preferred from the industrial viewpoint because use of expensive organic mineralizers is indispensable. The production of a high-silica mordenite without using an organic mineralizer has been tried. For example, the report of O. J. Whittemore, Jr., American Mineralogist, Volume 57, 1146-1151, 1972 discloses a process for preparing mordenite in which an alumina-silica gel having an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 10.1 to 15.5 is prepared from a dilute aqueous solution of sodium silicate, a dilute aqueous solution of aluminum chloride and hydrochloric acid; and the alumina-silica gel is crystallized by heating the gel at 184.degree. C. for 16 hours, whereby a mordenite having an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 12 to 19.5 is obtained. The product obtained by this process has low crystallinity and low purity. It is presumed that this is because the alumina-silica gel used for crystallization contains little or no sodium and is of no homogeneous phase. Japanese Unexamined Patent Publication No. 56-160316 discloses a process in which a high-silica mordenite having an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 15 to 25.8 is prepared by using an aqueous solution of sodium silicate and an aqueous solution of aluminum sulfate as starting materials. According to this process, sodium chloride is used in a large amount as an inorganic mineralizer for the synthesis of a high-silica mordenite. As the result of our investigations made on this process, it has been found that even if the synthesis is carried out faithfully according to the teaching of the above patent reference, a product cannot always be produced at a high efficiency, and that a high-silica mordenite having a high purity cannot be obtained unless reaction conditions such as the order of addition of the starting materials, the mixing speed, the stirring speed, the aging time and the crystallizing temperature are skillfully combined in a complicated manner.